Recoil-free fractions of iron in aluminous bridgmanite from temperature-dependent Mössbauer spectra

Abstract

Aluminous bridgmanite (Al-Bm) is the dominant phase in the Earth's lower mantle. In this study, the Mössbauer spectra of an Al-Bm sample Mg 0.868 Fe 0.087 Si 0.944 Al 0.101 O 2.994 were recorded from 65 to 300 K at 1 bar. The temperature dependence of the center shift was fitted by the Debye model and yielded the Debye temperatures of 305 ± 3 K for Fe 2+ and 361 ± 22 K for Fe 3+ . These values are lower than those of Al-free bridgmanite by 17 and 24%, respectively, indicating that the presence of Fe and Al increases the average Fe-O bond length and weakens the bond strength. At 300 K, the calculated recoil-free fractions of Fe 2+ (0.637 ± 0.006) and Fe 3+ (0.72 ± 0.02) are similar and therefore the molar fractions of Fe 2+ and Fe 3+ are nearly the same as the area fractions of the corresponding Mössbauer doublets. At 900 K, the calculated recoil-free fractions of Fe 3+ is 46% higher than that of Fe 2+ , implying that the molar fraction of Fe 3+ is only 41% for a measured spectral area fraction of 50%, and that the area fractions of iron sites may change with temperature without any changes in the valence state or spin state of iron. We infer that Fe 3+ accounts for 46 ± 2% of the iron in the Al-Bm and it enters the A site along with Al 3+ in the B site through the coupled-substitution mechanism. An Fe 2+ component with large quadrupole splitting (~4.0 mm/s) was observed at cryogenic conditions and interpreted as a high-spin distorted iron site.

Synthesis, Elasticity, and Spin State of an Intermediate MgSiO3‐FeAlO3 Bridgmanite: Implications for Iron in Earth's Lower Mantle

“…The spectrum at 35 GPa can be well explained by one HS Fe 3+ site, while the spectrum at 80 GPa needs at least two Fe 3+ sites for a satisfactory fitting. The fitted Quadrupole Splitting (QS) of Fe 3+ was consistent with those in previous studies (Lin et al, 2012; Liu et al, 2015). The LS proportion is 31%, consistent with XES measurement within uncertainties.…”

Stability of Fe,Al-bearing bridgmanite in the lower mantle and synthesis of pure Fe-bridgmanite

“…Nevertheless, on the basis of the summary of bridgmanite compositions known from the study by Liu et al . ( 31 ), it was reported that there are indications that an A-site vacancy is favored in Al-free bridgmanite, whereas an oxygen vacancy is dominant in Al-bearing bridgmanite.…”

Valence and spin states of iron are invisible in Earth’s lower mantle

“…Al 3+ facilitates Fe 3+ -enrichment in lower mantle Bdg through the coupled-substitution mechanism (Mg 2+ A + Si 4+ B = Fe 3+ A + (Fe 3+ , Al 3+ ) B ) (e.g., refs. 49 , 50 ). Whether Fe 3+ enters the B-site of Bdg through this coupled-substitution mechanism and further undergoes the spin transition in the lower mantle depends on the concentration of cations available to fill the B-site of Bdg and P–T conditions.…”